Comcast has punished some broadband Internet users by cutting off their service, saying they are looking out for their other 99.9 percent of customers who don’t send 18,000 e-mails a day, a company spokeswoman said Friday.

“Comcast has a responsibility to give (our customers) a good experience and address any issues that may impact that experience,” said Comcast spokeswoman Lorena Hernandez, who is based in San Ramon.

Comcast is contacting high-bandwidth users by phone and addressing their “excessive use,” she said. If their use can’t be curbed, Hernandez said that Comcast will help those users select a commercial-business product.

Someone who would be disconnected is termed an excessive user — someone sending 256,000 photos a month or

18,000 e-mails every hour, she said. According to Comcast, there are approximately

780,000 Bay Area Internet customers and less than 0.1 percent are considered transgressors.

“It shouldn’t be a problem if they are using the residential high-speed Internet as intended,” she said.

Comcast declined to reveal its download limits.

Adam Lazara, spokesman for Concord-based Astound Broadband, said there haven’t been any customers cut off for heavy use.

He said that unlike a one-speed-fits-all, Astound’s five-tiered broadband packages range from Low Speed to Power User Plus — eight megabits per second download at $64.95 a month — its highest use and most expensive tier. Astound serves about 20,000 customers in Contra Costa County.

“You would really need to be downloading about 1,000 music files a day, every day, for a month before we would even notice it,” Lazara said.

As Internet service providers try to keep up with the demand for increasingly sophisticated online entertainment such as high-definition movies, streaming TV shows andinteractive games, excessive use caps could become more common, some analysts said.

While some customers are unaware they are using so much capacity, sometimes because neighbors are covertly connecting through unsecured wireless routers, others may be running bandwidth-hogging business servers from their home, said Charlie Douglas, a spokesman for Comcast. The company says it gives customers a month to fix problems or upgrade to business accounts before shutting off their Internet service. It’s unclear how many customers have lost Internet service because of overuse. Cable companies are facing tough competition from telephone giants like AT&T and Verizon, which are installing new fiber-optic lines capable of carrying more Internet traffic.

The cable companies collectively spent about $90 billion in the past decade to improve their networks. And on cable networks, several hundred subscribers often share an Internet connection, so one high-traffic user could slow the rest of a neighborhood’s connections. Phone lines are run directly to each home, so a single bandwidth hog will not slow other connections. As Internet users make more demands of the network, cable companies in particular could soon end up with a critically short supply of bandwidth, according to a report released this month by ABI Research, a New York market-research firm. This could lead to a bigger crackdown on heavy bandwidth users, said the report’s author, Stan Schatt.

“These new applications require huge amounts of bandwidth,” he said. Cable “used to have the upper hand because they basically enjoyed monopolies, but there are more competitive pressures now.”

Companies have argued that if strict limits were disclosed, customers would use as much capacity as possible without tipping the scale, causing networks to slow.

Joe Nova of North Attleboro, Mass., lost Internet service after Comcast told him that he was using too much bandwidth to watch YouTube videos, listen to Internet radio stations and chat using a Web camera. He and other customers who complained of being shut off said they were not running servers from their homes.

“Sure, I’m online a lot, but there’s no way I could have been consuming that much capacity,” Nova said.

Other Internet service providers, including Time Warner Cable, Verizon and AT&T, say they reserve the right to manage their networks but have not yet suspended service. Some AT&T customers use disproportionately high amounts of Internet capacity, “but we figure that’s why they buy the service,” said Michael Coe, a spokesman for the company.

When Comcast canceled service to Frank Carreiro, who lives in a Salt Lake City suburb, he started a blog about the experience. His wife and six children then relied on sluggish dial-up Internet access until a phone company offered DSL service in his neighborhood. “For a lot of people, it’s Comcast or it’s nothing,” he said. Bob Williams, director of HearUsNow.org, a consumer Web site run by Consumers Union, said the vagueness of Comcast’s rules is “unfair and arbitrary.” “They’re cutting service off to the people who want to use it the most,” he said.